Ventilation + Filtration Calculator
Size the fresh-air ventilation this house needs against both a CO₂ target and the ASHRAE 62.2 minimum, then the filtration that closes the gap to a 4–6 ACHe clean-air target — and settle whether the unit should be an HRV or an ERV.
How it works. Enter the building size and the occupancy the house would hold with every bedroom full, then choose your targets. Everything below runs live off those numbers: the outdoor-air ventilation that holds indoor CO₂ where you want it, the same figure calculated the way ASHRAE 62.2 does it, the filtration needed to close the remaining clean-air gap, and — from the site's climate — whether that ventilation unit should recover moisture or reject it.
This is an early “ballpark” estimate to point you toward the right equipment — not a substitute for a mechanical design.
Ventilation
How much fresh air this house needs, and how the two accepted ways of working that out compare.
How much fresh air this house needs, and where that number comes from.
Enter whichever you have. Volume is what the filtration maths needs; floor area is what the code formula needs — we convert between them.
Fill this on the basis of every bedroom being full — the maximum number of people who would live here at one time, not who lives here today. Ventilation that only suits the current household has to be rebuilt when the household changes, and the cost of sizing up now is a slightly larger fan.
Indoor CO₂. CO₂ is a direct proxy for how much fresh air a space gets relative to how many people are in it. Lower targets require more outdoor air.
The first is the continuous flow rate your HRV or ERV has to deliver, running around the clock — not a boost setting, not a peak. The second is the size of unit to shop for. They are not the same number. A ventilation unit run permanently at its maximum is loud, short-lived, and rarely delivers its rated flow once real ductwork is attached, so continuous duty should sit at roughly two-thirds of rated capacity — which means specifying a unit around half again larger than the flow you need. That ratio varies by system: confirm it against the manufacturer's fan curve at your actual static pressure before ordering.
Size for the full house, then let controls scale the rate down when it is not full — see Controls in the next section.
The figure on the left is driven by occupancy — it knows nothing about the size of the building. ASHRAE 62.2 is the reverse: it counts floor area and bedrooms and knows nothing about how many people are actually in the house. Run both and take the higher. A large house with few people usually lands on the ASHRAE number; a small house with a full complement of people usually lands on this one.
This is not a mechanical design
Everything here is a ballpark, meant to point you at the right size and type of equipment. It assumes steady-state conditions and ignores air leakage, which varies enormously with a building's age and construction. Whether a unit ever delivers these flows comes down to duct design, static pressure, and commissioning — none of which a calculator can see. Have a qualified mechanical designer confirm the numbers before anything is ordered.
Specifying the unit
Three decisions that matter more than the brand on the box.
Mechanical configuration
There are three ways to install an HRV or ERV, and they are not equivalent.
- Simplified
- Both the fresh-air supply and the stale-air pickup tie into the furnace return. Cheapest to install. Fresh air only moves when the air handler runs, it arrives pre-mixed with whatever the return is already carrying, and you have no say in which rooms receive it.
- Return ducted
- Dedicated stale-air pickups from the wet rooms, but fresh air is still delivered into the furnace return for distribution. Better control over what leaves the house. Still no control over where fresh air lands.
- Fully ducted · recommended
- Dedicated supply ducts to every bedroom and living space, dedicated exhaust from bathrooms, kitchen and laundry, working independently of the heating system. It costs more, and it is the only configuration that lets you decide where fresh air goes and what pressure each room sits under — which is the entire reason for installing one.
Room pressure is the part most often skipped. Supplying bedrooms slightly positive and extracting from the wet rooms means moisture, odours and combustion by-products travel away from where people sleep rather than toward them. A simplified installation cannot do that at any price.
Controls — let the house ask for what it needs
The flow rate above is sized for the house at its fullest: every bedroom occupied, everyone home. Most of the time it is not that house. Running that rate continuously through an empty afternoon costs heat, costs electricity, and in winter strips moisture out of a house that did not need it stripped.
- CO₂ sensing
- CO₂ tracks occupancy almost perfectly — it rises when people are in a room and falls when they leave. A controller that modulates fan speed against a CO₂ setpoint delivers the full design rate when the house is full and backs off when it is not, without anyone thinking about it. The real advantage is that it responds to things nobody would ever set a schedule for: it eases off when a window is open and the outdoors is already doing the work, and it ramps up on the evening you have twenty people over for dinner — the one night the house genuinely needs every cfm it was sized for. Put sensors where people actually spend hours: bedrooms first, then the main living space.
- Humidity sensing
- The other half of the same idea. Boost on a humidity rise so showers and cooking are cleared at the moment they happen, and — in a cold climate — allow the rate to ease back when indoor humidity is already at the low end, rather than ventilating the house further into dryness. This is what keeps the winter figures in section 03 from being the whole year's story.
- What not to do
- Do not use a controller as a reason to buy a smaller unit. Size for the full house, then let the controls reduce the rate. A unit that cannot reach the design flow has no headroom to give back.
Filtration inside the unit
The recovery core is not a filter, and the coarse screens shipped with most units are there to protect the machine, not the people in the house.
- MERV-13 · always
- Run MERV-13 on the outdoor-air intake continuously, all year round. It is the grade at which a filter starts capturing the fine particulate that matters for health rather than just the lint that matters for the fan.
- MERV-15 or better · wildfire
- Step up during wildfire smoke events, then back down afterwards. Higher grades cost more pressure drop than a ventilation unit wants to carry permanently, so treat them as an event response rather than a default.
Specify the unit's fan against the pressure drop of a loaded MERV-13 rather than a clean one, and put the filter somewhere a person will actually be willing to reach. A filter that is awkward to change is a filter that does not get changed.
HRV or ERV
The recovery question, settled from this site's climate rather than a rule of thumb.
The recovery question, answered from your climate rather than a rule of thumb.
Why this is not obvious. An energy-recovery ventilator moves moisture across a membrane from whichever airstream has the higher vapour pressure to the lower one — so its effect reverses with the season. In a humid summer it keeps outdoor moisture out. In any heating season it keeps indoor moisture in. Whether that helps or hurts depends entirely on where you are.
Filtration
What ventilation cannot do on its own — and the part people most often skip.
What ventilation cannot do on its own.
Why this matters more than it sounds. Ventilation is sized against CO₂, and CO₂ is a gas — dilute it and it goes. Particles do not behave that way. Wildfire smoke, cooking particulate, road soot, pollen, dust and the fine fraction that reaches deep into the lung are all brought in by outdoor air as readily as they are carried out by it. On a bad air day, ventilation is the delivery mechanism.
The measure that covers both is ACHe — equivalent air changes per hour, counting outdoor air and filtered air together. Healthy Buildings (Allen & Macomber) puts the useful range at 4–6, and 4 is the right starting point for most homes. Fresh-air ventilation alone almost never reaches it: the flow that holds a good CO₂ level is typically well under one air change an hour. Filtration closes the rest of the gap, and it does so without a heating penalty, which is why it is usually the cheaper half of the answer.
It only counts if the outdoor air is clean. Leaked air arrives through cracks, not through a filter. On a rural site with good air, that exchange genuinely dilutes what the house generates. In a city, beside a highway, or anywhere with a wildfire season, it is delivering the particulate you are trying to remove — and the credit above should be treated as zero.
A leaky house is not a healthier house. Air moving through an assembly carries moisture into it. In a heating climate that warm, humid indoor air reaches the cold side of the wall and condenses there, which is how walls rot from the inside. Leakage that helps your air-change number can be quietly destroying the building.
This is the real trade-off of modern construction. Airtight is better: it puts you in control of where air enters, what it passes through on the way in, and how much heat it takes with it on the way out. The price is that a tight house needs mechanical ventilation and needs more filtration, because it has stopped getting either by accident.
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Spread them out. Several smaller units distributed through the space consistently outperform one large unit of the same total rating, because clean air has to reach the room a person is in. Bedrooms first — that is where the longest continuous exposure happens.
Ducted filtration in the air handler can carry some or all of this instead, but only while the air handler is running. If it cycles with the thermostat, it is not delivering the clean-air rate above.
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Find my next step →Disclaimer. This calculator is not a replacement for a qualified mechanical engineer or designer, and is not intended to be the sole resource for sizing ventilation or filtration equipment. It is an early “ballpark” estimate. Other factors specific to your building — air-tightness, occupancy type, temperature, dew point, and VOCs — must be considered for a holistic approach to healthy indoor air. The imperial and metric results are computed from whichever unit and entry mode you select for building size.